The present invention relates generally to chain saws and, in particular, to a drive sprocket assembly for driving a saw chain.
In Swedish Patent Application 9302842-1 (PCT WO95/06549) there is shown a drive sprocket device for chain saws, where the saw chain runs around a loose outer drive ring rolling on an inner drive rim sprocket. A rim sprocket is characterized by having an outer periphery with an annular row of recesses which can receive the radially inner ends of the drive links of the saw chain. The width of the recesses only slightly exceeds the thickness of the drive links, and the recesses on each side are bordered by smooth cylindrical surfaces of the rim sprocket. With this device the saw chain when it is moving will always be automatically tensioned with a force proportional to the cutting force. This reduces wear and improves safety.
In older chain saw designs with gears between the motor and the drive sprocket, the sprocket was usually of a rim type and fastened to the end of the drive shaft without axial mobility (play), as disclosed in U.S. Pat. No. 2,884,798. Newer chain saws with direct drive have the usual rim sprockets arranged to be axially slidable on a hollow spline shaft fastened to the clutch drum, allowing a certain axial mobility of the clutch drum, and permitting the use of guide bars of different thicknesses or having some deformation. The hollow shaft is jounaled on the drive shaft and axially secured with a locking ring on the drive shaft end.
To allow the use of lightweight high speed motors without excessive chain speed, the drive sprocket diameter has to be small, which has resulted in the need to reduce the size of the splines. It is thus difficult to make splines with sufficient strength, if rim wheels are to be used.